1. Field of Invention
The present invention relates to a voltage compensating circuit. More particularly, the present invention relates to a voltage compensating circuit for a sensorless type DC brushless motor.
2. Description of Related Art
In the field of motors, the three-phase brushless motor is a kind of motor apparatus with high power efficiency; moreover, the volume thereof can be reduced more easily than that of other kinds of motor. Therefore, the DC brushless motor is well suited to every kind of 3C production and is now in widespread use.
The three-phase DC brushless motor is a kind of electrical commutation motor (ECM). The basic design of the three-phase DC brushless motor is determining the position of the armature in the motor at first, then appropriately varying the excitation on the armature to induce a magnetic field with different directions, and then the armature can be driven by the induced magnetic field. That is to say, a driving circuit varies the direction of the induced magnetic field as long as the position of the armature is changed, and then the induced magnetic field drives the armature again. The armature will thus continuously rotate when the action above-mentioned is continued. According to the directional track of the varied magnetic field, it can be seen that the magnetic field induced by the armature is a rotating magnetic field.
According to the basic design, only the position of the armature is precisely determined, the direction of the induced magnetic field can be correctly determined, and the efficiency of the motor can be maintained. Hence, a position detector is usually attached with the conventional three-phase brushless DC motor for determining the position of the armature. In consideration of the tendency towards production miniaturization and cost reduction, a technique has been developed to determine the position of the armature without the position detector, that is, a sensorless detection technique.
The present sensorless detection technique comprises an indirect induced potential detection and a direct induced potential detection. The operation of the indirect induced potential detection includes fetching the voltages from the three terminals and the neutral point at first, transmitting the voltages into a filter, a voltage attenuator and a position detector in order, and generating a commutation signal. The induced potential obtained by the indirection induced potential detection has some problems, including a low signal-to-noise ratio (SNR) and delay; hence, the position of the armature cannot be precisely determined. Therefore, a direct induced potential detection was developed.
The difference between direct induced potential detection and indirect induced potential detection is that the direct induced potential only fetches the voltages from the three terminals, and then transmits them into a voltage clamper for overcoming the problems of the indirect induced potential detection. But an error will be caused by the voltage drops on the switch and the diode devices in the driving circuit of the DC brushless motor, and the error will cause jittering and efficiency degradation when the DC brushless motor is operating.
FIG. 1 shows a basic equivalent circuit of a conventional three-phase DC brushless motor apparatus. The three-phase DC brushless motor apparatus comprises a driving circuit and a motor. A power supply VI is used to provide the current needed by the operation of the motor. Switch devices S1–S6 and diode devices D1–D6 constitute the driving circuit of the three-phase brushless DC motor. Induction coils LA–LC, resistor RA–RC and induced potential eA–eC respectively indicate the induction coils of the A, B and C phases on the armature in the motor, equivalent resistances, and the induced potential induced by the induction coils. Moreover, a neutral point is formed by connecting one terminal of A, B and C phase to each other.
The switch devices S1–S6 are connected with a controlling signal. The purpose of the controlling signal is to turn on and turn off the switch devices S1–S6 in order to enable the current provided by the power supply VI to pass continuously and transiently through the two of the induction coils LA–LC, and thereby to excite and produce a magnetic field to rotate the armature of the motor. Hence the direction of the magnetic field is determined by the direction of the current, the input phase of the current is determined by the upper arm switch devices S1, S3, and S5 and the output phase of the current is determined by the lower arm switch devices S2, S4, and S6.
Therefore, position detection is defined as detecting the present position the armature in the motor for determining the direction of the magnetic field needed by the armature to be rotated to the next position, and can determine which switch devices of the switch devices S1–S6 should be turned on to excite the induction coils on the armature to produce the magnetic field with a desired direction.
In the aspect of the above-mentioned direct induced potential detection, an induced voltage VN the neutral point NP is an important parameter. The voltage VN can be obtained by measuring the terminal voltages of the phase not being excited, because, in an excitation, the current passes through only two coils. Because the induced voltage value on the neutral point is desired, the measuring should be carried out in a transient without current. Referring to FIG. 1, the induced coils LB and LC are excited by the current provided by the power supply VI, and the current passes through the turned on switch devices S3 and S6. Then, in the moment the switch device S3 is turned off, induced potentials eB, eC, and an induced current I are produced from induced coils LB and LC because the current provided by the power supply VI cannot flow into the armature in the motor.
The induced current. I passes through the loop comprising the switch device S6 and the diode device D4; therefore, according to the principals of circuit analysis, the ideal induced voltage VN at the neutral point is:VN=eA/2Thus, a voltage VA can be measured on a node NA:VA=3eA/2that is to say, the desired neutral voltage VN can be measured via the voltage VA on the node NA.
In general, voltage drops of voltage VS and VD respectively appear at the switch device S6 and diode device D4 when the induced current I passed through; therefore, the induced voltage VN on the neutral NP in practice is:VN=(eA/2)+[(VS−VD)/2]Also, the voltage VA measured at the node NA becomes:VA=(3eA/2)+[(VS−VD)/2]It can be seen that an error of (VS−VD)/2 occurs, and the error causes a jitter and low efficiency.